The reverse TCA cycle is a series of chemical reactions by which organisms produce carbon compounds from carbon dioxide and water. The reverse TCA cycle requires electron donors and often times, bacteria will use hydrogen, sulfide or thiosulfate for this purpose. The reverse TCA is considered to be an alternative to photosynthesis which produces organic molecules as well.
This process requires a number of reduction reactions using various carbon compounds. The enzymes, unique to reverse TCA, that function in catalyzing these reactions include: ATP citrate lyase, 2-oxoglutarate:ferredoxin oxidoreductase, and pyruvate:ferredoxin oxidoreductase. ATP citrate lyase is the enzyme responsible for cleaving citrate into oxaloacetate and acetyl CoA. These enzymes are unique to reverse TCA and are necessary for the reductive carboxylation to occur.
An example of a microorganism that utilizes reverse TCA includes Thermoproteus. Thermoproteusis type of prokaryotic that is characterized as a hydrogen-sulfur autotroph. The organisms classified as Thermoproteus utilizes sulfur reduction for metabolic processes. As previously mentioned, organisms that use reverse TCA may use sulfur as an electron donor to carry out this metabolic process. The acetyl-CoA pathway utilizes carbon dioxide as a carbon source and often times, hydrogen as an electron donor to produce acetyl-CoA.
Describe the role of the carbon monoxide dehydrogenase and acetyl-CoA synthetase in the acetyl-CoA pathway. The acetyl coenzyme A CoA pathway, commonly referred to as the Wood-Ljungdahl pathway or the reductive acetyl-CoA pathway, is one of the major metabolic pathways utilized by bacteria. This specific pathway is characterized by the use of hydrogen as an electron donor and carbon dioxide as an electron acceptor to produce acetyl-CoA as the final product.
Acetyl-CoA is a major component in numerous metabolic processes as it plays a key role in the citric acid cycle. The main function of acetyl-CoA in the citric cycle is to transport carbon atoms. In regards to molecular structure, acetyl-CoA functions as the thioester between conezyme A and acetic acid.
Specific types of organisms that utilize this pathway include archaea classified as methanogens and acetate-producing bacteria as well. The following is a brief overview of the acetyl-CoA pathway.. The acetyl-CoA pathway begins with the reduction of a carbon dioxide to carbon monoxide. The other carbon dioxide is reduced to a carbonyl group. The two major enzymes involved in these processes are carbon monoxide dehydrogenase and acetyl CoA synthase complex. The carbon dioxide that is reduced to a carbonyl group, via the carbon monoxide dehydrogenase, is combined with the methyl group to form acetyl-CoA.
The acetyl-CoA synthase complex is responsible for this reaction. Carbon monoxide dehydrogenase, the enzyme responsible for the reduction of a carbon dioxide to a carbonyl group, functions in numerous biochemical processes. These processes include metabolism of methanogens, acetogenic and sulfate-reducing bacteria. Specifically, the acetyl-CoA pathway is utilized by bacteria that are classified as methanogens and acetate-producing organisms.
The carbon monoxide dehydrogenase allows organisms to use carbon dioxide as a source of carbon and carbon monoxide as a source of energy. The carbon monoxide dehydrogenase can also form a complex with the acetyl-CoA synthase complex which is key in the acetyl-CoA pathway.
Acetyl-CoA synthetase is a class of enzymes that is key to the acetyl-CoA pathway. The acetyl-CoA synthetase functions in combining the carbon monoxide and a methyl group to produce acetyl-CoA.. The ability to utilize the acetyl-CoA pathway is advantageous due to the ability to utilize both hydrogen and carbon dioxide to produce acetyl-CoA. Specific types of bacteria which utilize the acetyl-CoA pathway include methanogens and acetate-producing bacteria. Methanogens are types of organisms, classified as archaea, that exhibit the ability to produce methane as a metabolic byproduct.
Methanogens, which are found in numerous environments including wetlands, marine sediments, hot springs and hydrothermal vents, are able to use carbon dioxide as a source of carbon for growth. In addition, the carbon dioxide is used as an electron acceptor in the production of methane. Methanogens are able to utilize the acetyl-CoA pathway to fix carbon dioxide.
Acetate producing bacteria, or acetogens, are a class of microorganisms that are able to generate acetate as a product of anaerobic respiration. This process, known as acetogenesis, will occur in organisms that are typically found in anaerobic environments. Acetogens are able to use carbon dioxide as a source of carbon and hydrogen as a source of energy. The 3-hydroxypropionate cycle is a carbon fixation pathway that results in the production of acetyl-CoA and glyoxylate.
Carbon fixation is a key pathway in numerous microorganisms, resulting in the formation of organic compounds deemed necessary for cellular processes. One of the pathways that is utilized for carbon fixation is the 3-hydroxypropionate cycle. Specifically, in this cycle, the carbon dioxide is fixed by acetyl-CoA and propionyl-CoA carboxylases. This process results in the formation of malyl-CoA which is further split into acetyl-CoA and glyoxylate.
Propionyl-CoA carboxylase is an enzyme that functions in the carboxylation of propionyl CoA. This enzyme functions in the mitochondrial matrix and is biotin dependent. The acetyl-CoA carboxylase utilized in this cycle is biotin-dependent as well and catalyzes the carboxylation of acetyl-CoA to malonyl-CoA. This pathway produces pyruvate via conversion of bicarbonate and also results in the production of intermediates such as acetyl-CoA, gloxylate and succinyl-CoA.
To date, this pathway has been identified in organisms classified as green non sulfur bacteria, specifically Chloroflexus aurantiacus and in chemotrophic archaea. The green non sulfur bacteria uses reduced sulfur compounds, such as hydrogen sulfide or thiosulfate as an electron donor for metabolism.
The ability of Chloroflexus aurantiacus to utilize this pathway is unique. The 3-hydroxypropionate cycle is a newly discovered pathway, thus, the exact details involving this process in regards to enzymes and intracellular components are still currently under investigation. However, the cycle can be broken down into two major phases, carbon dioxide fixation and glyoxylate assimilation.
The electron transport chain in the mitochondrial membrane is not the only one that generates energy in living cells. In plant and other photosynthetic cells, chloroplasts also have an electron transport chain that harvests solar energy. Even though they do not contain mithcondria or chloroplatss, prokaryotes have other kinds of energy-yielding electron transport chains within their plasma membranes that also generate energy.
When energy is abundant, eukaryotic cells make larger, energy-rich molecules to store their excess energy. The resulting sugars and fats — in other words, polysaccharides and lipids — are then held in reservoirs within the cells, some of which are large enough to be visible in electron micrographs.
Animal cells can also synthesize branched polymers of glucose known as glycogen , which in turn aggregate into particles that are observable via electron microscopy. A cell can rapidly mobilize these particles whenever it needs quick energy.
Athletes who "carbo-load" by eating pasta the night before a competition are trying to increase their glycogen reserves. Under normal circumstances, though, humans store just enough glycogen to provide a day's worth of energy. Plant cells don't produce glycogen but instead make different glucose polymers known as starches , which they store in granules.
In addition, both plant and animal cells store energy by shunting glucose into fat synthesis pathways. One gram of fat contains nearly six times the energy of the same amount of glycogen, but the energy from fat is less readily available than that from glycogen.
Still, each storage mechanism is important because cells need both quick and long-term energy depots. Fats are stored in droplets in the cytoplasm; adipose cells are specialized for this type of storage because they contain unusually large fat droplets. Humans generally store enough fat to supply their cells with several weeks' worth of energy Figure 7. Figure 7: Examples of energy storage within cells. A In this cross section of a rat kidney cell, the cytoplasm is filled with glycogen granules, shown here labeled with a black dye, and spread throughout the cell G , surrounding the nucleus N.
B In this cross-section of a plant cell, starch granules st are present inside a chloroplast, near the thylakoid membranes striped pattern.
C In this amoeba, a single celled organism, there is both starch storage compartments S , lipid storage L inside the cell, near the nucleus N. Qian H. Letcher P. A Bamri-Ezzine, S. All rights reserved. This page appears in the following eBook. Aa Aa Aa. Cell Energy and Cell Functions. Figure 3: The release of energy from sugar. Compare the stepwise oxidation left with the direct burning of sugar right. Figure 5: An ATP molecule. ATP consists of an adenosine base blue , a ribose sugar pink and a phosphate chain.
Figure 6: Metabolism in a eukaryotic cell: Glycolysis, the citric acid cycle, and oxidative phosphorylation. Glycolysis takes place in the cytoplasm. Cells need energy to accomplish the tasks of life. See also: Lipid ; Lipid metabolism ; Triglyceride triacylglycerol. It should also be pointed out that amino acid oxidation is intermediate in its O 2 requirement between glycolysis and mitochondrial fatty-acid oxidation because some reduced cofactors are produced in the cytosol and others are produced in the mitochondria.
See also: Amino acid ; Amino acid metabolism. The other consideration that guides the magnitude of a cellular O 2 requirement is the degree to which a cell is busy with reactions that demand the hydride carried on NADH and NADPH and whether reducing equivalents can be produced cytosolically. Unlike a fireplace, whose purpose is to combust fuel fully to generate heat Fig. Thus, the logic of life is such that the relatively low energy electrons carried on cytochrome C in the inner mitochondrial membrane have much less power to do meaningful work than the electrons carried on cytosolic NADPH.
The former can donate to O 2 to generate water, having already generated a proton gradient in the descent from the high-energy state in NADH to the low-energy state in reduced cytochrome C. The latter can donate electrons to beta-keto groups and alkenes to perform reductive biosynthesis. Therefore, it would be illogical for cells to let electrons flow downhill too far if they are needed for biosynthetic reactions.
One of the best examples of a set of metabolic pathways that minimizes respiration occurs in white adipocytes fat-storing cells , which are specialized to convert glucose to triglycerides Fig.
This begins with import of glucose and conversion to pyruvate in the cytosol. In the mitochondria, pyruvate is converted to oxaloacetate and Ac-CoA by pyruvate carboxykinase and pyruvate dehydrogenase. These products are condensed to form citrate, which is then exported to the cytosol for conversion to cytosolic Ac-CoA and oxaloacetate.
The glucose-derived Ac-CoA is not oxidized to CO 2 in the citric acid cycle, but rather is effectively exported to the cytosol to produce fat. Moreover, because the adipocyte cytoplasm can produce NADPH by running the oxidative and nonoxidative phases of the pentose phosphate pathway and by converting oxaloacetate to malate and then malate to pyruvate, it has a system to capture most of glucose's available electrons into fat synthesis without a high oxygen demand.
Although it is beyond the scope of this article to cover cell replicative and anabolic pathways, it is important to consider that every cell and tissue make everything in the human body from food using metabolic transformations whose biosynthetic complexities greatly exceed the catabolic complexities of breaking down carbohydrates, fats, and proteins. Gluconeogenesis, ketogenesis, amino acid synthesis, nucleic acid synthesis, and steroid synthesis depend on reduced cofactors. See also: Fat and oil ; Nucleic acid ; Steroid.
Like de novo lipogenesis, cytosolic ROS detoxification depends on NADPH that can be produced in the cytosol by nonaerobic processes, including the pentose phosphate pathway. Similarly, in brown adipocytes, which express high levels of the proton pore—forming uncoupling protein, high levels of oxygen consumption are linked to heat production rather than ATP formation Fig.
It could be, then, that bacteria can't expand in cell and genome size because they can't physically associate the right set of genes with their energetic membranes. If that's the case, the acquisition of mitochondria and the origin of complexity could be one and the same event. The question is, what kind of a cell acquired mitochondria in the first place? Most large-scale genomic studies suggest that the answer is an archaeon — that is, a prokaryotic cell that is in most respects like a bacterium.
That begs the question, how did mitochondria get inside an archaeon? The answer is a mystery but might go some way toward explaining why complex life derives from a single common ancestor, which arose just once in the 4 billion years of life on Earth. Peter Mitchell's demonstration that ATP synthesis is powered by proton gradients was one of the most counterintuitive discoveries in biology, and it took a long time to be accepted.
The precise mechanisms by which a proton gradient is formed and coupled to ATP synthesis chemiosmotic coupling is now known in atomic detail, but the broader question that drove Mitchell — why are proton gradients so central to life? Recent research suggests that proton gradients are strictly necessary to the origin of life and highlights the geological setting in which natural gradients form across membranes, in much the same way as they do in cells. But the dependence of life on proton gradients might also have prevented the evolution of life beyond the prokaryotic level of complexity, until the unique chimeric origin of the eukaryotic cell overcame this obstacle.
Allen, J. The function of genomes in bioenergetic organelles. Efremov, R. Nature , — doi Lane, N. How did LUCA make a living? Chemiosmosis in the origin of life. Bioessays 32 , — doi The energetics of genome complexity. Nature , Martin, W. On the origin of biochemistry at an alkaline hydrothermal vent. Mitchell, P. Coupling of phosphorylation to electron and hydrogen transfer by a chemi-osmotic type of mechanism.
What Is a Cell? Eukaryotic Cells. Cell Energy and Cell Functions. Photosynthetic Cells. Cell Metabolism. The Origin of Mitochondria. Mitochondrial Fusion and Division. The Origin of Plastids. The Origins of Viruses.
Discovery of the Giant Mimivirus. Volvox, Chlamydomonas, and the Evolution of Multicellularity. Yeast Fermentation and the Making of Beer and Wine.
Dynamic Adaptation of Nutrient Utilization in Humans. Nutrient Utilization in Humans: Metabolism Pathways. An Evolutionary Perspective on Amino Acids. Mitochondria and the Immune Response. Stem Cells in Plants and Animals. Promising Biofuel Resources: Lignocellulose and Algae. The Discovery of Lysosomes and Autophagy. The Mystery of Vitamin C. By: Nick Lane, Ph.
Citation: Lane, N. Nature Education 3 9 The proton gradients that power respiration are as universal as the genetic code itself, giving an insight into the origin of life and the singular origin of complexity.
Aa Aa Aa. How Cells Breathe. Fine Details. Figure 2: The structure of complex I, the largest protein complex involved in respiration in bacteria and mitochondria, as revealed by X-ray crystallography. The structure a suggests the piston mechanism shown b , whereby shunting the piston drives protons across the membrane through three separate channels.
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